This invention relates to pedal assemblies for vehicle braking systems of the brake-by-wire type of the kind in which operation of a brake pedal is adapted to cause actuation of the brakes of a vehicle fitted with an Electronic Braking System (EBS).
In an EBS for a vehicle there is usually only an electrical connection between the brake pedal and an Electronic Control Unit (ECU) which is adapted to control the braking system in accordance with the signals received from the pedal assembly. Usually the pedal assembly is provided with a displacement transducer, normally a potentiometer, adapted to produce a varying electrical output proportional to the driver""s braking demand. This usually works in tandem with a spring adapted to provide the driver with a degree of mechanical feedback indicative of the magnitude of the applied demand. Such an arrangement is shown in EP-A-0 195 579.
The arrangement disclosed in EP-A-0 195 579 incorporates a mechanism by which the driver is able to generate a braking demand signal for use by the EBS system. In such an arrangement the overriding intention is to provide such a demand signal at the expense of any improvement in driver feel or comfort. As a result the xe2x80x9cfeelxe2x80x9d of the pedal and associated system is generally unsatisfactory.
A good conventional hydraulic braking system is arranged to provide the driver with a comfortable pedal feel, action, and braking response. This xe2x80x9cfeelxe2x80x9d comprises at least three distinct components, although any combination of two of these would also be effective. Firstly the driver feels a change in system stiffness as the demand or pedal displacement increases. Secondly, the driver feels damping in the pedal action as the pedal is moved progressively through its travel. Finally the driver experiences hysteresis in the response of the system as the pedal is applied and released. These three elements are, within a conventional system, generated by a combination of hydraulic restrictions and damping as well as actuator stiffness and built-in hysteresis.
According to our invention in a pedal assembly of the kind set forth feedback reaction means is provided so that feedback to the driver through the pedal provides a comfortable pedal feel.
For example, the pedal feel may be similar to that achieved in a conventional hydraulic braking system fitted to a non-EBS equipped vehicle.
Spring means and damping means may be built into the braking system in order to provide the driver with a comfortable pedal feel.
This can be achieved by a spring assembly in combination with a damper, for example an elastomeric reaction disc or an hydraulic damper, either incorporated in the spring assembly or comprising a rotary dashpot which may be fitted to the pivot pin for an angularly moveable lever carrying the foot pad.
The spring assembly may comprise a two-rate spring assembly or a variable rate spring assembly.
In a modification the EBS includes a hysteresis function.
In one construction this comprises a filter adapted to provide a fixed hysteresis value or a value adapted in dependence upon either the rate of change of the brake application, the magnitude of the brake application, or a combination of both.
The pedal assembly may be embodied in a pedal box which accommodates the feedback reaction means.
In another construction the pedal acts on the piston of an hydraulic master cylinder through a push-rod and a variable rate spring, and a linear displacement transducer is adapted to produce a signal to an ECU in response to displacement of the pedal in a brake applying direction.
In this construction the hydraulic master cylinder builds in hydraulic damping, and hysteresis, by virtue of it connecting to the pedal.
According to another aspect of our invention a pedal assembly of the kind set forth is adapted to provide two signal inputs for a main electronic control unit which, in turn, is adapted to service at least two sub-systems of a braking system.
The signals may be sensed through two potentiometers, either rotary or linear in construction.
When the potentiometers are of the rotary type, they may be connected to the pedal through a ratio arm. Hence pedal travel in the form of pedal rotation is sensed.